JxL – 2:1 is the classic gearing, but some people don’t have the strength / fitness for it immediately. If you have a 32t chainring (try and fine one that’s not ramped – a DH one is good) then 16t sprocket at the back will give you 2:1. I’ve known people to run 32/17 and 32/18 as well.
You can also go for bigger sprockets and chainrings to get the same ratio, but better pickup and lower wear. For example, I run a 36t chainring and 18t sprocket to give 2:1, but slower wear.
If you have a normal rear wheel with a freehub, you can use spacers and a sprocket to find the right ratio for you. I seem to remember some of the singlespeed kits come with16t and 18t sprockets. If you’re really cunning, you can run both sprockets at once, so it’s easy to swap between ratios if you’re getting knackered or have a long road section. Then you could also get one of those rear mech things… 😉
Basically, if you’re already a fairly strong rider, then 16t is as good a place as any to start. Expect to blow up occasionally, and also to push up at least a couple of hills on the first few rides / weeks / months.